Rechargeable (or secondary) lithium ion batteries (hereinafter lithium ion batteries) are commonly used today in, for example, hand held (cellular) telephones and laptop computers, among other things. Those lithium batteries are favored because of their high energy density, high voltage, and good charge retention. These batteries typically use a lithiated carbon material as the negative electrode, intercalation compounds, such as transition metal oxides (e.g., LixCoO2), as the positive electrodes, microporous polyolefin membranes as the separator between the electrodes, and liquid electrolyte contained within the pores of the membrane and in electrochemical communication between the electrodes (reference to positive and negative is during discharge). Further detail on the materials of construction of these various components may be found in Linden, D., Editor, Handbook of Batteries, 2nd Edition, McGraw-Hill, Inc., New York, N.Y. 1995, pp. 36.1-36.77, and Besenhard, J. O., Editor, Handbook of Battery Materials, Wiley-VCH Verlag GmbH, Weinheim, Germany, 1999, for example pp. 47-55, each of which is incorporated herein by reference. These batteries are distinguished from so called “lithium polymer” batteries that are characterized by electrolytes that are in the form of a gel or solid, and consequently, have lower conductivities. Some such separators are described in Besenhard, J. O., Ibid., pp. 557-558, incorporated herein by reference.
When the lithium ion battery is fully charged, the positive electrode (cathode) becomes a strong oxidizing agent because of its high positive valence, thereby creating at the positive electrode/separator interface a very tough environment for the battery components (electrodes, electrolytes, and separators). All these components are susceptible to degradation, via oxidation, in this environment.
Oxidation of the separator is undesirable. The separator serves several functions, one is to insulate the electrodes from one another, i.e., prevent internal shorting. This insulating function is accomplished by the use of polyolefin membranes. When a polyolefin separator is oxidized, it looses its physical and chemical integrity and is thus unsuitable for its original intended function. This shortens the useful life of the battery because the battery no longer can hold its charge due to internal shorting within the battery.
This oxidative environment at the positive electrode/separator interface may be more fully understood with reference to the following.
For example, a typical lithium ion battery may have: a positive electrode (cathode) containing lithium cobalt oxide, lithium nickel oxide, or lithium manganese oxide (LixCoO2 will be discussed hereinafter); a negative electrode (anode) containing a lithiated carbon; a liquid electrolyte containing a lithium salt (e.g., LiPF6 or LiClO4) in an aprotic organic solvent (mixtures of EC, DEC, DMC, EMC, etc); and a microporous polyolefin membrane. During discharge, lithium ions migrate from the negative electrode (anode) containing the lithiated carbon to the positive electrode (cathode) containing LixCoO2. The cobalt is reduced from a +4 valence to a +3 valence, and current is generated. During charging, current is supplied to the battery at a voltage in excess of the discharge voltage to move the lithium that migrated to positive electrode (cathode) back to negative electrode (anode), and the cobalt is oxidized from the +3 valence to the +4 valence state. In commercial batteries, a fully charged battery typically consists of about 75% of the cathodic active material (e.g., cobalt) to be at the +4 valence state and, if LixCoO2 is used, x is about 3.5. In this state, the cobalt of the positive electrode (cathode) is a strong oxidizing agent. It can and will attack materials around it, particularly the separator.
Separator oxidation can be seen. FIG. 1 is a photograph of the magnified image of an oxidized separator. The separator is a microporous polyethylene membrane made by a ‘wet’ or ‘phase inversion’ process. This separator was recovered from a fully charged cell that had been stored in an oven (85° C.) for three days. The dark areas are the oxidized areas. FIG. 2 is a schematic illustration of the cross section of the membrane shown in FIG. 1. It is believed that these dark areas have less physical and chemical strength. Poor mechanical strength can lead to shorts and thus battery failure.
The foregoing oxidation problem is common. When batteries are stored in a fully charged condition, when batteries are stored, at temperatures greater than room temperatures, in a fully charged condition, or when batteries are charged at a constant voltage ˜4.2V for an extended period of time, the oxidation problem arises. The latter situation is common, for example, when a laptop computer is left ‘plugged in’ and therefore continuously charging. In the future, the oxidation problem may become more severe. The current trend is for these batteries to be able to operate at temperatures greater than room temperature and for these to be stored, fully charged, at temperatures greater than room temperature. Therefore, oxidation at these greater potential temperatures will likely be more severe.
Accordingly, there is a need for batteries and separators that resist oxidation at the positive electrode (cathode)/separator interface of a lithium ion battery.